Gas turbines have traditionally used diffusion flame combustion chambers because of their reliable performance and reasonable stability characteristics. However, as a result of the high temperatures involved during combustion, this type of combustion chamber may produce unacceptably high levels of nitrogen oxide pollutants called NOX. Due to increasingly strict regulation on pollutant emissions, industrial power generation manufacturers have turned to low emission technology, and many new power plants now employ low emission gas turbine engines. These gas turbines achieve low NOX emission by using Lean Pre-Mixed (LPM) combustion. In these systems, the fuel (typically natural gas) is mixed with a relatively high proportion of air before burning. The thermal mass of the excess air present in the combustion chamber absorbs the heat generated during combustion, thus limiting the temperature rise to a level where thermal NOX is not formed.
While lean premixed combustion has demonstrated significant reduction in NOx emissions, LPM combustion may suffer from combustion instabilities due to the lean nature of the fuel flow in that operating range. This phenomenon is also known as combustion dynamics.
With lean premixed fuel, the combustion flame burns on the border of not having enough fuel to keep burning, and a phenomenon analogous to a flickering flame takes place, giving rise to pressure fluctuations. These pressure fluctuations excite the acoustic modes of the combustion chamber resulting in large amplitude pressure oscillations. The oscillations produced travel upstream into the fuel nozzle and create an oscillating pressure drop across the fuel injectors. This may result in an oscillatory delivery of fuel to the combustion chamber. When the oscillating fuel-air mixture burns in the combustion chamber, the flame area fluctuates giving rise to heat release oscillations. Depending upon the relative phasing of these heat release oscillations and the acoustic waves, a potentially self-exciting feedback loop may be created giving rise to oscillations whose amplitude grows with time. These oscillations typically occur at discrete frequencies that are associated with natural acoustic modes of the combustion chamber and its higher order harmonics thereof.
Such combustion driven instabilities have adverse effect on the system performance and operating life of the combustion chamber. The oscillations and their resultant structural vibrations can cause fretting and wearing at the walls of the combustion chamber, reducing high cycle fatigue life and affecting the overall performance.
Accordingly, there exists a need for methods and systems providing combustion dynamics reduction. There exists a further need to simultaneously reduce the sensitivity to fuel composition.